The goals of this project are to examine the interactions between type C murine retra-viruses and their host cells both in vivo and in cell culture. Particular emphasis is currently being placed in (1) the possible normal functions of xenotropic virus during pregnancy and embryonic development, and (2) the evaluation of physiological and other factors as they influence the susceptibility of the host to tumorigenesis upon exposure to tumor viruses and virus-inducing carcinogens. Virological and biochemical methodology are principally employed in this study. Interferon and other anti-viral agents are employed as reagents.